The present invention relates to an arrangement for enabling rotation movement between the tandem or caterpillar axle and body of a vehicle.
One significant field of application for this type of solution is a stabilization system for stabilizing the lateral roll of the cabin of a vehicle with articulated steering, for instance. One vehicle group of this type is found in forest machines, the drivers of which often are, due to their working conditions in a swaying vehicle, the recipients of a lot of low-frequency (0.5 to 10 Hz) vibration with a randomly varying amplitude that has been found hazardous to health. The ill effects of such a vibration to the driver are tiredness, lower work efficiency, and stress directed to the musculoskeletal system and spine; especially the lower back. The effect of this vibration on a person's health has been discussed by Griffin (1990) and Dupruis & Zerlett (1986), among others. In the case of a forest machine and especially a forwarder, the crosswise movements, or lateral sway, in relation to the longitudinal axis of the machine are known to be the most hazardous. The sways are typically caused by a wheel or caterpillar of the forwarder on one side going over a high obstacle, such as rock or stump. These sways are typically the stronger, the more uneven the terrain is and the higher the velocity of the vehicle is.
The swaying of the vehicle affects not only the comfort of the driver, but also the stability of the vehicle and especially the terrain-friendliness of the forest machine. By damping the sways, it is possible to increase the velocity of the vehicle, which affects directly the output of the vehicle.
The above-mentioned drawbacks have been solved in different ways in the present forest machines, for example. A simple way is to replace a two-wheel axle with a specific rocker tandem axle that due to the gearing effect between the wheels evens out the sway of the vehicle to some extent, since the rocker tandem axle tends to substantially halve the sway caused by an obstacle. On the other hand, this solution leads to getting two smaller sways instead of just the one depending on the size and shape of the obstacle, which is not welcome as such. Accelerations directed to the driver may also be reduced by using an active suspension developed by the car industry, for example. However, this type of active suspension does not eliminate the low-frequency lateral sways that are directed to the cabin, in particular, and that are the most hazardous to health according to studies.
Forest machines mostly have a two-body structure, in which case the front and rear parts are connected to each other by a joint with two degrees of freedom that makes the steering of the machine possible and conventionally also permits these body parts to rotate in relation to each other, if the axles of both parts are fixed stationary to the body parts. Another conventional arrangement is to arrange the axle of one body to be a rocker axle, if a degree of freedom permitting the rotation of the bodies substantially in relation to the longitudinal axis of the machine has not been implemented in the joint between the bodies of the machine.
In mechanical power transmission, both ordinary and rocker tandem axles are generally used. Rocker tandem axles are usually fixed to the body. A tandem axle distributes the attractive force and surface pressure more evenly to the terrain and evens out the sway to some extent. In any case, a tandem axle typically allows for a better advancement in slope terrain and in poorly bearing terrain conditions due to a larger contact surface and lower surface pressure. By means of the tandem axles, the centre of gravity of the machine can also be made lower without essentially compromising the ground clearance, and this affects advantageously to the stability of the machine. Ordinary two-wheel axles can be fastened both fixedly and in an articulated manner to the body, i.e. as a swing axle. The second axle is usually a fixed one, while the other swings.
In published patent EP 0 824 413, the swinging movements of the vehicle cabin is controlled by a device, in which an axle that substantially swings in relation to the longitudinal axis of the vehicle is arranged to the front body that is arranged rotatingly to the rear body. The front body also has at least one actuator for controlling this swing axle. The shift of the centre of gravity of the front body in a transverse direction to the longitudinal axis of the front body causes in the actuator a push or pull movement that evens out the shift of the centre of gravity, until balance is achieved in the actuator and the front body is in a substantially horizontal position. This way, the lateral roll of the front body caused by the unevenness of the terrain can be reduced or eliminated entirely. The mass to be moved by this type of device is relatively small, so the need for hydraulic power required in the device is not unreasonably high.
The device described above works well when controlling a swing axle, but it cannot be applied as such to controlling wheels installed on a tandem axle. Because the use of tandem axles in forest machines continues to grow, the control of the swaying movements related to them has also become more important than before.
Known solutions do enable keeping the driver in the cabin that is orientated to be continuously substantially horizontal. However, when the stabilizing axle is located high, it is not possible to avoid a lateral translation movement, which is caused by the above-mentioned lateral swing or roll, acting on the driver. This is a problem in several known stabilizing arrangements for forest machine cabins.